Rumours of further delays in the launch of Windows XP's successor Vista are dismissed by Office 2007 product manager Darren Strange, who says guardedly that dates have already been set. It is due for release to enterprises in 'er, November or December' and to the retail market in January.

This means, as we have pointed out before, that the operating systems on PCs sold over Christmas will be out of date shortly after the Christmas fairy has gone back into hibernation for another year. It will be a case of 'On the 25th day of Christmas, my true love said to me, time to upgrade from Windows XP.'

Buyers are already becoming cannier in the timing of their purchases, aware of the insanity of rushing to buy in December before prices fall in the January stock clearances, and  Microsoft will not be the only company to fear the January Vista launch will hit seasonal sales.

Our man in Silicon Valley reports today that Microsoft is considering issuing coupons for Vista upgrades with PCs bought from October, which seems a sensible move.

But could it be that Microsoft also fears competition from the resurgent Apple, which has just previewed Leopard, the next generation of the MacOS? Apple is in a very strong position to win sales of home computers from Microsoft, especially when PCs are offering only a relatively old operating system.

The desktop version of MacOS X and Windows XP were both launched in 2001, but the Apple software has gone through four major upgrades since then and the final pre-Leopard version makes XP look more then a little dated.

Apple will also have the advantage of launching Leopard after Vista. Chief executive Steve Jobs said during last week's Leopard preview that he was holding a lot of features back in case they get copied. This cuts both ways, of course, because if Microsoft springs some surprises in the final release of Vista, Apple can ensure Leopard matches or even trumps them.

A final Apple advantage, at least during the transition phase when the computing world moves over to the new software, is its oldest one: the closed Mac architecture. This is what gave Apple its reputation for reliability, because the company could ensure its software and hardware worked together whereas Windows had to be all things to all machines.

Apple's crafty move to the Intel platform eroded the price advantage that economies of scale gave the PC and still allows it to tune its software to its own hardware.

Of course Microsoft has overwhelming strengths in the number and variety of Windows applications, and its penetration of IT infrastructure. But Apple stands to make a fortune if it shaves even a few points off Microsoft's market share.